Lead frame support plate and window clamp for wire bonding machines

ABSTRACT

A lead frame support plate  200  and a window clamp  400  for wire bonding machines are disclosed herein. In a described embodiment, the lead frame support plate  200  includes a network of suction grooves  218  provided on a support surface  212 , each suction groove  218  being arranged to be in fluid communication with at least a vacuum hole  216  to enable a suction force to be created, in response to a vacuum force, along the network of suction grooves for holding a lead frame against the support surface. A window clamp  400  having slots for compensating deformation of the window clamp and a method of fabricating the lead frame support plate are also disclosed.

BACKGROUND AND FIELD

This invention relates to a lead frame support plate and window clamp for wire bonding machines.

To ensure proper and accurate wire bonding, a lead frame needs to be held in place with respect to a wire bonding machine and FIG. 1 illustrates a conventional approach. FIG. 1 shows part of a wire bonding machine 100 comprising a window clamp 102 placed over a strip of lead frame 104 and a lead frame support plate 106 for supporting the strip of lead frame 104. The window clamp 102 includes two support sections 108 which are held by corresponding window clamp holders 110 for applying a clamping force to the window clamp 102.

FIG. 2 is an enlarged side perspective view of the part of the wire bonding machine 100 of FIG. 1 without showing the window clamp holders 110. The wire bonding machine further includes a heater block 112 for heating the lead frame support plate 106 and thus, the strip of lead frame 104 during the wire bonding process.

FIG. 3 is a perspective view of the lead frame support plate 106 of FIG. 2 which includes a series of elongate side bores 114 and a multiplicity of micro-holes 116 (normally more than 1000 micro-holes 116) across a major part of the lead frame support plate 106. The micro-holes 116 are in fluid communication with corresponding side bores 114 to create a network of air channels. When the side bores 114 are sealed up and a vacuum force is applied at the base of the lead frame support plate 106, a balanced suction force is created via the micro-holes 116 at the top surface of the lead frame support plate 106 to maintain the position of the strip of lead frame 104 during the wire bonding process. However, forming of the micro-holes 114 is an extremely time-consuming process.

Further, referring to the arrangement illustrated in FIG. 2, it has been found that with the lead frame support plate 106 and the strip of lead frames 104 heated to a high temperature (for example, 200° C.), the window clamp 102 is subjected to uneven heat distribution because the window clamp's lower surface which is in contact with the strip of lead frames 104 would have a higher temperature than the window clamp's upper surface since the upper surface is exposed. This uneven heat distribution together with the clamping force asserted by the window clamp holders 110 would cause the window clamp 102 to deform or warp. FIG. 4 shows an exemplary window clamp 300 which has deformed due to the uneven heat distribution. Such deformation would result in loose contact of the window clamp with leads of a lead frame resulting in floating leads or pads due to an insufficient clamping force therebetween. The bonding quality would thus be affected.

It is an object of the present invention to provide a lead frame support plate and window clamp which addresses at least one of the disadvantages of the prior art and/or to provide the public with a useful choice.

SUMMARY

In accordance with a first aspect, there is provided a lead frame support plate for a wire bonding machine; the lead frame support plate comprising a support surface for supporting a lead frame; an array of vacuum holes through which a vacuum force is to be applied; a network of suction grooves provided on the support surface, each suction groove being arranged to be in fluid communication with at least a said vacuum hole to enable a suction force to be created, in response to the vacuum force, along the network of suction grooves for holding the lead frame against the support surface.

An advantage of the described embodiment is that with the suction grooves, fabricating time of the lead frame support plate is greatly reduced and yet the suction grooves are able to achieve a balanced suction force on the support surface for holding the lead frame.

Preferably, the network of suction grooves includes a combination of linear and diagonal suction grooves. The suction grooves may be hemispheric and may have a maximum depth of about 0.026 mm. Preferably, the lead frame support plate further comprises a series of side bores extending between a first edge to a second edge of the support plate's body; the series of side bores arranged to intersect with respective vacuum holes.

According to a second aspect, there is provided a method of fabricating a lead frame support plate for a wire bonding machine, the lead frame support plate comprising a body having a support surface for supporting a lead frame and an array of vacuum holes through which a vacuum force is to be applied, the method comprising forming a network of suction grooves on the support surface with each suction groove being in fluid communication with at least a said vacuum hole to enable a suction force to be created, in response to the vacuum force, along the network of suction grooves for holding the lead frame against the support surface.

With such a fabrication method, the number of vacuum holes which needs to be formed is greatly reduced since a balanced vacuum force for holding the lead frame is provided by the network of suction grooves.

The method may also comprise forming the array of vacuum holes. Preferably, the method may further comprise forming a series of side bores which extends between a first edge to a second edge of the support plate's body; wherein the series of side bores are arranged to intersect with respective vacuum holes.

In a third aspect, there is provided a window clamp for a wire bonding machine, comprising an array of wire bonding windows having corresponding apertures configured to register with respective semiconductor dies, the array of wire bonding windows having two furthermost windows along a longitudinal axis; and a pair of slots disposed on each side of the array, each slot having an elongate portion defined by a first end and a second end, the first and the second ends extending at least between the apertures of the two furthermost windows.

An advantage of the described embodiment is that with pair of slots, it is possible to compensate for deformation of the window clamp caused by uneven heat distribution and clamping force asserted to sides of the window clamp during wire bonding. This thus achieves a more effective clamping of leads of a lead frame by the window clamp.

Preferably, each slot is generally C-shape with arm portions extending respectively from the first and second ends and generally orthogonal to the elongate portion. This configuration has been found to be more effective in compensating for deformation or warpage of the window clamp.

The first and second ends may extend to farthest edges of the apertures of the two furthermost windows. Preferably, the array of wire bonding windows includes a plurality of rows of windows, and wherein a first row and a last row of the plurality of rows respectively includes the two furthermost windows.

The apertures and the corresponding wire bonding windows may be of the same size. Alternatively, the size of the wire bonding windows is greater than the corresponding apertures.

The lead frame support frame of the first aspect and the window clamp of the second aspect may form part of a wire bonding machine independently or used together.

It should be appreciated that features relating to one aspect may also be applicable to the other aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 shows part of a wire bonding machine having a conventional window clamp and conventional lead frame support plate, including window clamp holders;

FIG. 2 is an enlarged side perspective view of the part of the wire bonding machine of FIG. 1 (without showing the window clamp holders);

FIG. 3 is a perspective view of the conventional lead frame support plate of FIG. 2;

FIG. 4 shows effects of uneven heat distribution and clamping force to a window clamp as experienced by the window clamp of FIG. 2;

FIG. 5 is a top view of a lead frame support plate 200 having an array of vacuum holes in fluid communication with a network of suction grooves, according to a preferred embodiment;

FIG. 6 includes FIGS. 6 a and 6 b in which FIG. 6 a is an enlarged view of Detail W of FIG. 5; and FIG. 6 b is an enlarged cross-sectional view of FIG. 6 a in direction N-N;

FIG. 7 is an enlarged view of Detail X of FIG. 5 to show the vacuum holes and the suction grooves more clearly;

FIG. 8 includes FIGS. 8 a and 8 b in which FIG. 8 a is a side view of the lead frame support plate of FIG. 5 when used to support a strip of lead frame; and FIG. 8 b is an enlarged view of portion A of FIG. 8 a to illustrate air vacuum flow through the suction grooves;

FIG. 9 is a perspective view of a window clamp according to the preferred embodiment;

FIG. 10 is a top view of the window clamp of FIG. 9; and

FIG. 11 shows a variation of the window clamp of FIG. 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 5 shows a lead frame support plate 200 of a preferred embodiment of the present invention. The lead frame support plate 200 includes a generally rectangular body 202 having first and second edges 204, 206 and side sections 208, 210 which define a boundary for a support surface 212 and a bottom surface 214 (see FIG. 8 a). Size of the support surface 212 is adapted to accommodate or support a strip of lead frames similar to what is shown in FIGS. 1 and 2.

A series of side bores (not shown) of diameters about 1.0 mm-1.5 mm are formed using an endmill similar to what is illustrated in FIG. 3. The elongate side bores extend through the body 202 from the first edge 204 to the second edge 206, and the number of side bores are fewer than that illustrated in FIG. 3, for reasons which will be apparent later. The side bores may not only be vertical ones and diagonal side bores are also envisaged.

Next, an array of vacuum holes 216 are created which are through holes of diameter of about 0.15 mm extending from the support surface 212 to the bottom surface 214 and which intersects the side bores to form a network of air channels (not shown). The vacuum holes 216 of the array are regularly spaced and evenly spread over the support surface 212 and this is shown more clearly in FIG. 7.

A network of suction grooves 218 is next formed on the support surface 212 and each suction groove 218 is hemispheric and has a maximum depth of about 0.026 mm. Thus, in this embodiment, the suction grooves 218 may also be called “micro-grooves” due to its dimension. FIG. 6 a is an enlarged view of Detail W of FIG. 5 to show the suction grooves 218 more clearly, and FIG. 6 b is an enlarged cross-sectional view in the direction N-N of FIG. 6 a.

Each suction groove 218 is elongate and extends linearly (i.e. vertically or horizontally) or diagonally to form a crisscross network illustrated in FIG. 5, and each suction groove 218 is configured to be in fluid communication with at least one vacuum hole 216 and this is shown more clearly in FIG. 7. When a vacuum force is applied via the vacuum holes 216, this creates suction on the support surface 212 and specifically along the suction grooves 218 to create a linear suction force. FIG. 8 including FIGS. 8 a and 8 b illustrate the effects of the vacuum force when the lead frame support plate 200 is used during wire bonding. FIG. 8 a is a side view of the lead frame support plate 200 of FIG. 5 arranged to support the strip of lead frame 104 shown in FIGS. 1 and 2. Similar to the prior arrangement of FIG. 2, the body 202 of the lead frame support plate 200 is arranged to transfer heat from a heat source to heat the strip of lead frame 104.

It would be appreciated that the suction grooves 218 create air gaps between the strip of lead frame 104 and the support surface 212 of the lead frame support plate 200 and when a vacuum force 220 is applied through a vacuum aperture 222, the vacuum force 220 draws or sucks out air linearly from the suction grooves 218 in the direction of arrows 224 (see FIG. 8 b) to create a balanced vacuum holding force between the strip of lead frame 104 and the support surface 208. The direction of suction is from the support surface 212 to the bottom surface 214 via the suction grooves 218 and the vacuum holes 216 (i.e. through the network of air channels) and through the vacuum aperture 222. In this way, the strip of lead frame 104 is held stationary with reference to the support surface 208 and thus, the lead frame support plate 200 during the wire bonding.

With the use of suction grooves 218, the number of vacuum holes 216 needed to create a balanced vacuum force is far fewer than the micro-holes 116 in the prior support plate 106 of FIG. 3. As an example, for a similar size lead frame support plate, the number of vacuum holes 216 needed to create a balanced vacuum holding force on the support surface 212 may just be around 100, compared to 1000 micro-holes 116 required in the support plate 106 of FIG. 3. With the number of vacuum holes 216 reduced, this also reduces the number of end bores needed to create the network of air channels. As a result, this significantly reduces the time required for fabricating the support plate 200. Moreover, the use of the suction grooves 218 also enables the support plate 200 to handle different types of lead frames.

FIG. 9 shows a window clamp 400 according to the preferred embodiment of the invention. The window clamp 400 is similar to the one 102 illustrated in FIG. 1 except that the window clamp 400 includes an array of wire bonding windows 402 having four columns 404 and ten rows 406. Each wire bonding window 402 includes a corresponding aperture 408 arranged to register with a semiconductor die for the wire bonding. In this embodiment, the size of the wire bonding window 402 is greater than the aperture 408 but it should be appreciated that this may vary depending on the type of window clamp. For example, the size of the wire bonding window 402 may be the same as the aperture 408.

The wire bonding windows 402 of each column 404 of the array are arranged along a respective longitudinal axis 410. The ten rows 406 include a first row 406 a and a last row 406 b and the wire bonding windows 402 along respective longitudinal axis in the first and last rows 406 a, 406 b are the furthermost wire bonding windows. Taking a first longitudinal axis 410 a of the array as an example, the wire bonding window 402 a in the first row 406 a and the wire bonding window 402 b in the last row 406 b are considered the furthermost wire bonding windows 402 a, 402 b along the first longitudinal axis 410 a.

Further, unlike the window clamp 102 of FIG. 1, the window clamp 400 includes a pair of slots 412, 414 for compensating thermal expansion of the window clamp. The width of each slot is about 3 mm. Each slot 412, 414 is generally C-shape and has an elongate portion 416, 418 having ends 417, 419 which extend at least between the apertures 408 a, 408 b of the two furthermost wire bonding windows 406 a, 406 b. In other words, the length of the elongate portion 416, 418 should be at least distance D of FIG. 10 which is the distance between the two nearest edges of the apertures 408 a, 408 b of the two furthermost wire bonding windows 402 a, 402 b along the first longitudinal axis 410 a. In this way, the elongate portion would transverse the entire length of the wire bonding windows 406. The distance D may be about 70.5 mm.

Preferably, the length of the elongate portion 416, 418 is not more than distance Z which is the two farthest edges of the apertures 408 a, 408 b of the two furthermost wire bonding windows 402 a, 402 b. The distance Z may be about 80.0 mm.

Each slot 412, 414 also includes two arm portions 420, 422 orthogonal to respective elongate portions 416, 418 and which extends away from the array of wire bonding windows 402 to form the C-shape. As it can be appreciated from FIG. 10, the arm portions 420, 422 of each slot 412, 414 extend in a direction opposite to each other.

When support sections 424, 26 of the window clamp 400 are held by window clamp holders 110, the applied clamping force creates component forces CF at the four corners of the arm portions 420, 422 of the respective slots 412, 414 as illustrated in FIG. 9 and clamping forces CLF at the support sections 424, 426. Since the support sections 424, 426 deform more in the Y direction, components of the clamping force CLF would force the window clamp 400 to counter the deformation shown in FIG. 4 and thus, the window clamp 400 would remain substantially flat.

With the window clamp 400 as proposed, clamping performance is improved and the leads of lead frames are clamped more firmly.

The described embodiments should not be construed as limitative. For example, other shapes and sizes of the lead frame support plate may be used and not necessary that shown in FIG. 5. Also, the size and pattern of the network of suction grooves 218 may be modified according to the type of lead frame strip. The diameters of the vacuum holes and the end bores may be varied according to application too.

The number and size of the wire bonding windows 402 of the window clamp may be varied too, depending on the type of window clamp. Further, the wire bonding windows 402 may not be arranged in straight lines as shown and other arrangements may be possible. Further, the size and shape of the slot 412, 414 may be varied too and FIG. 11 shows a variation 500 of the window clamp 400 of FIG. 10. In the window clamp 500 of FIG. 10, instead of C-shape slots, the window clamp 500 includes a pair of elongate slots 502, 504 with ends 502 a, 502 b, 504 a, 504 b that extend between respective apertures 506, 508 of the furthermost bonding windows 510, 512 along corresponding longitudinal axis 514, 516. Such slots 502, 504 may also help to compensate the deformation or warp but may not be as preferred as the C-shape slots 412, 414 of FIG. 10.

It should be appreciated that the lead frame support plate 200 and the window clamps 400, 500 in the described embodiment may be used independently of each other and need not necessary be used in the same wire bonding machine.

Having now fully described the invention, it should be apparent to one of ordinary skill in the art that many modifications can be made hereto without departing from the scope as claimed. 

The invention claimed is:
 1. A wire bonding machine comprising: a lead frame support plate configured and operative to support a lead frame when the wire bonding machine performs wire bonding, the lead frame support plate comprising: a support surface for supporting the lead frame; an array of vacuum holes configured to apply a vacuum force; a network of suction grooves provided on the support surface, each suction groove being arranged to be in fluid communication with at least one vacuum hole to enable a suction force to be created, in response to the vacuum force, along the network of suction grooves for holding the lead frame against the support surface; a window clamp positioned over the lead frame support plate for clamping the lead frame on the support surface of the lead frame support plate, the window clamp comprising an array of wire bonding windows having corresponding apertures configured to register with respective semiconductor dies arranged on the lead frame; wherein the array of wire bonding windows of the window clamp comprises: two furthermost windows along a longitudinal axis; and a pair of slots positioned on each side of the array, each slot comprising an elongate portion defined by a first end and a second end, the first and the second ends extending at least between the apertures of the two furthermost windows.
 2. The wire bonding machine according to claim 1, wherein the network of suction grooves includes a combination of oblique suction grooves forming a criss-cross network, the oblique suction grooves forming oblique angles with respect to an edge of the support surface, and non-oblique suction grooves forming non-oblique angles with respect to the edge of the support surface and the plurality of oblique suction grooves.
 3. The wire bonding machine according to claim 1, wherein the suction grooves are hemispheric.
 4. The wire bonding machine according to claim 1, wherein the suction grooves have a maximum depth of about 0.026 mm.
 5. The wire bonding machine according to claim 1, further comprising a series of side bores extending between a first edge to a second edge of the support plate's body, wherein the side bores are arranged to intersect with respective vacuum holes.
 6. The wire bonding machine according to claim 1, wherein each slot is generally C-shape with arm portions extending respectively from the first and second ends and generally orthogonal to the elongate position.
 7. The wire bonding machine according to claim 1, wherein the first and second ends extend to farthest edges of the apertures of the two furthermost windows.
 8. The wire bonding machine according to claim 1, wherein the array of wire bonding windows includes a plurality of rows of windows, and wherein a first row and a last row of the plurality of rows respectively includes the two furthermost windows.
 9. The wire bonding machine according to claim 1, wherein the apertures and the corresponding wire bonding windows are the same size.
 10. The wire bonding machine according to claim 1, wherein the size of the wire bonding windows is greater than the corresponding apertures. 